In recent years, there has been a demand for higher output or performance in an internal combustion engine such as a gasoline engine. Further, in recent years, lower fuel consumption has been also demanded. When engine performance is improved in order to handle such a demand, a temperature of a combustion chamber increases, and hence a thermal load on an exhaust valve particularly increases.
Here, there is proposed a valve (a hollow poppet valve) in which a hollow part is formed from a head of a poppet valve having a stem and a head integrated with each other to the stem so as to decrease the weight of the valve and for example, coolant (refrigerant) such as metallic sodium having high thermal conductivity is enclosed in the hollow part along with an inert gas so as to improve the thermal conductivity of the valve (hereinafter, also referred to as a heat transfer effect of the valve). In such a valve, heat generated in a combustion chamber in accordance with the starting of an engine is actively transferred through the valve.
For example, Patent Literature 1 discloses a hollow valve for an internal combustion engine in which a hollow hole is drilled so as to be blocked at a head face along the axis of the stem from the head to the stem. In the hollow valve, a head face part is provided with a through-hole which is continuous to the hollow part and has a diameter smaller than the maximal inner diameter of the hollow part, a sealing member is fitted and adhered to the through-hole, and a boundary between a valve body and the sealing member is located near the center of the valve axis so as to avoid an acute part to which stress is likely to be concentrated. Accordingly, the durability and the reliability of the hollow valve are improved.
Further, Patent Literature 2 discloses a hollow valve for an internal combustion engine in which a cooling medium is enclosed in a hollow hole drilled from a head to a stem and an unevenness part is provided in at least an inner peripheral face of the hollow hole. Accordingly, the heat transfer efficiency to the cooling medium near the head is improved and hence the thermal load of the head is remarkably decreased when the valve is operated.
Meanwhile, even in a valve seat insert for a valve opening and closing an intake and an exhaust of an internal combustion engine, there has been a demand for keeping excellent cooling performance capable of suppressing an increase in peripheral temperature of a combustion chamber by radiating heat from the valve as well as wear resistance.
For such a demand, for example, Patent Literature 3 discloses a valve seat insert for an internal combustion engine which is formed of iron base sintered alloy and in which two layers of a seat-face side layer and a valve contact face side layer provided with a valve contact face are integrated with each other, the valve contact face side layer is 10 to 45% by a volume % with respect to the entire valve seat insert, a boundary face between the valve contact face side layer and the seat-face side layer preferably has an average angle of 20 to 90° with respect to the valve seat insert axis, and the boundary face is adjusted to ±300 μm at the average position in the height direction. Accordingly, it is possible to obtain a valve seat insert having excellent wear resistance and high thermal conductivity.